Method of die casting high melting point alloys



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4 I BY United States Patent 3,528,478 METHOD OF DIE CASTING HIGH MELTING POINT ALLOYS Byron W. Koch, Bruno Sachs, and William D. Navarro, Toledo, Ohio, assignors to National Lead Company, New York, N.Y., a corporation of New Jersey Filed July 25, 1968, Ser. No. 747,589 Int. Cl. B22b 17/04 U.S. Cl. 164-113 Claims ABSTRACT OF THE DISCLOSURE The invention discloses a method of die casting high temperature alloys such as ferrous alloys. The shot sleeve of a die casting machine is provided with a protective thermal barrier prior to pouring the charge thereinto. The shot plunger is protected by the insertion of a seal, and the active portion of the shot sleeve is made larger in diameter than the elfective diameter of the shot plunger so that a residual metal annulus remains in the shot sleeve after injection. The metal annulus, and seal are removed as a part of the biscuit with the casing. A further feature of the invention is the provision of a floating shot sleeve that has clearance for relative movement with respect to the die blocks.

BACKGROUND OF THE INVENTION At the present time the die casting process is used extensively for the casting of parts from alloys of zinc, aluminum, magnesium and certain copper alloys such as brass. The patented art contains many references to the casting of ferrous alloys, but little, if any, commercial application of the die casting process to these alloys has been made. Not only is it extremely difficult to con struct dies for the casting of metals having a melting point upwards of 2000 F., but given a satisfactory and workable die, the present machines will operate only a few cycles without freezing. It is apparent, then, that improvements must be made in the method of operating the injection portion of a die casting machine if the advantages inherent in the die casting process are to be realized with metals having a relatively high melting point. The present invention represents such improvement.

The invention is based on the premises that the metal of the charge must be introduced into a shot sleeve with some means to prevent adherence of this metal to the wall of the sleeve; that the shot plunger should not come into direct contact with the metal to be injected into the die; that the apparatus should force into the die only that portion of the charge which remains in a liquid state since there will always be some instantaneous freezing of the charge adjacent the walls of the shot sleeve even if protected; and that in certain installations in which a die is made of a highly refractory metal the shot sleeve assembly must be arranged to expand and contract, in response to thermal forces, independently of the adjacent die parts.

The invention is shown in the accompanying drawings in conjunction with a die casting machine having several conventioal parts. In the drawings:

FIG. 1 is a fragmentary side elevational view, with parts in section, showing die casting apparatus for carrying out the method of the present invention;

FIG. 2 is a fragmentary vertical sectional view of the machine of FIG. 1 with the dies opened and a portion of the shot sleeve being protected;

FIG. 3 is a view similar to FIG. 2 with the shot sleeve being filled with metal;

FIG. 4 shows the parts of FIGS. 2 and 3 with the .dies closed and the shot plunger advanced to make a casting;

FIG. 5 shows the parts of FIGS. 2 and 3 with the casting and biscuit ejected from the dies; and

FIG. 6 is an enlarged fragmentary sectional view of a shot sleeve and plunger.

In the drawings, 10 designates a vertical die casting machine which is essentially conventional and comprises a frame 11 having a bed 12 on which a cover die 13 is fixed in the usual manner. Tie bars 14 carry the upper operating elements of the machine, including a sliding plate 15 to which is bolted an ejector cylinder 16 to operate an ejector plate 17. An ejector die half 18 is also carried by the sliding plate 15 and is moved to and from mating position with the cover die 13, by conventional toggles (not shown).

Suitable impression blocks 20 and 2.1 form a die cavity 22 to which metal enters via a gate 23 from a shot sleeve 24. The shot sleeve 24 projects upwardly through the cover die 13 and in one form of the apparatus has a clearance fit (as indicated in FIG. 6) to allow the sleeve to expand and contract relative to the cover die 13 for reasons hereinafter discussed.

A shot plunger 25 is provided which is operated by a conventional shot cylinder 26 when metal is to be injected into the die. Normally, the shot plunger lies in a retracted position, being advanced to a first position when a shot is made, and being further advanced to a second position to push the biscuit out of the end of the shot sleeve.

The apparatus used to carry out the method of the present invention differs in one respect from conventional apparatus in that the shot sleeve 24 is preferably stepped and has an enlarged active upper portion of a diameter D (FIG. 2). The remainder of the shot sleeve is sized to accommodate the shot plunger 25 at a diameter designated d in FIG. 2. For convenience, the enlarged portion of the shot portion of the shot sleeve into which a protective element is inserted and into which the charge of metal is poured will be hereinafter referred to as a shot chamber 31. If the stepped configuration of the shot sleeve is not used, the protective cup hereinafter described will be inserted directly over the plunger 25 or directly over a seal 32 if this element is made a part of the casting apparatus.

The shot plunger 25 has a normal fit with the interior of the lower portion of the shot sleeve in the smaller diameter area, but according to the present invention the shot plunger never comes into contact with the charge of metal. To this end, a seal 32 is provided to overlie a reduced diameter tip 33 of the plunger 25. The seal 32 may take many suitable forms, but the form shown in FIGS. l-5 may be preferred. As there indicated the seal is a cup-shaped member having a depending flange 34 that may engage the interior of the shot sleeve 24 either with a tight fit ahead of the shot plunger or may be tapered to fit the shot sleeve loosely at its upper end and tightly only adjacent the lower end of the flange 34 of the cup.

Another suitable form of seal is shown in FIG. 6 in which a cylindrical expendible seal 35 is provided ahead of the shot plunger 25, as will be later described in more detail.

The method of the present invention may be followed from FIGS. 2-5, inclusive. Commencing with the dies 13 and 18 in their open position, the first step is to position a seal such as seal 32 over the tip of the shot plunger 25. The next step is to provide a heat barrier and protective element on the inside of the shot chamber 31. In a very simple execution of the present invention, a cup 36 made of asbestos paper is dropped into the shot chamber 31 over the seal 32. The asbestos paper cup can be quite thin and has been found to be capable of retaining the charge of molten ferrous metal very adequately without disintegrating. The walls of the shot chamber may also be protected by spraying the interior surface of the shot chamber with a slurry of silica flour, bentonite and water. This slurry is known in the art of centrifugal casting of ferrous metals as a refractory mold wash. The refractory mold wash adheres better to a hot sleeve than to a cold one. Satisfactory adherence will be obtained after one or two shots have been made, but it may be desirable to preheat the shot chamber 31 prior to an initial operation. The mold wash serves not only as a thermal barrier, but prevents metal from adhering to the wall of the shot chamber and thus facilitates removal of the biscuit as hereinafter described.

When the shot chamber 31 has been suitably protected, metal can be poured as indicated diagrammatically in FIG. 3. After pouring the charge of metal, the die halves are brought to closed position and the shot plunger 25 advanced to force metal into the die as indicated in FIG. 4. It is important to note from this figure that only the center portion of the metal is injected, leaving an annulus represented by the difference in diameter D of the shot chamber and the small diameter d of the seal 32 which forms the elfective head of the shot plunger. It will also be noted that the surface of the shot plunger does not come into direct contact with the residual portion of the charge in the shot chamber. The remaining fibers from the asbestos paper cup 36 will, for the most part, stay with the residual metal in the annulus.

A predetermined time after the shot is made, the dies are opened and the completed casting ejected as indicated in FIG. 5. As the dies first open the shot plunger is advanced to push the casting away from the cover die and the biscuit clear of the shot sleeve, and is next retracted to the position shown. The usual ejector pins operate to free the casting from the ejector die half. The biscuit is broken off from the casting at the gate and may be remelted. Release of the residual metal from the shot cylinder is made easier by the asbestos cup 36 or by the coating which was applied thereto in the initial i step above described.

After the casting has been removed completely from the machine, a new seal 32 is dropped into the shot sleeve, a new protective procedure is established for the shot sleeve interior and the process repeated.

Certain modifications of the invention will be apparent. One or all of the different expedients now to be described may be adopted. First, the cast ferrous metal seal 32 may be replaced with a cylindrical length of tubing 35. The lower end of the tubing rests directly on the upper end of a shoulder 53 on the shot plunger 25 and the open upper end of the tubing is closed off by a cap 54. The cap 54 is preferably made from a refractory material such as carbon, although in some instances a cap of the same ferrous alloy as is used for the casting may be adopted.

In the casting of ferrous alloys, it is necessary to use die blocks of refractory metals in place of conventional steels. These refractory metals are brittle and diflicult to work, and also have a different coeflicient of thermal expansion than adjacent parts as the shot sleeve. An important aspect of the present invention is shown in somewhat enlarged and exaggerated form in FIG. 6. As there shown, the shot sleeve 24 is sized to have a clearance space .56 where is would normally have a slip fit with the adjacent die blocks. There is also a clearance 4 57 for longitudinal freedom. A split ring 58 is provided to prevent gross displacement of the shot sleeve 24 relative to the die.

Most importantly, the shot sleeve is provided with a lip extension 24a to overlie the inner end of the die block opening in which the shot sleeve is received. The lip 24a has a tapered periphery that cooperates with a corresponding taper 24b in the ejector die to center the shot sleeve. Closure of the dies plus the force imposed by the injected metal keep the lip 24a firmly against the adjacent surface of the die block during casting. The lip 24a further acts to trap flash that might form as a result of a shot. Flash entering under the lip 24a is broken away from the casting as the casting is ejected because the direction of ejection is perpendicular to the plane in which the flash might form, and the force required to break off the flash is taken by the steel shot sleeve and is not imposed on the brittle metal of the die blocks.

The present invention has been disclosed in conjunction with a preferred apparatus, but other modifications than those described may be required. The invention makes possible the economical die casting of ferrous alloys or alloys of metals having a high metal point as contrasted to known die casting of metals melting in the range of aluminum, brasses, etc.

We claim:

1. The method of die casting relatively high melting point metals such as ferrous alloys in a die casting machine having a shot plunger that reciprocates in a shot sleeve to force metal into a die cavity, which method includes (A) providing a metal receiving portion of the shot sleeve with a protective thermal barrier,

(B) charging metal into the protected portion of said shot sleeve,

(C) advancing the shot plunger to inject into the die only a center portion of the charged metal leaving a residual metal annulus in the shot sleeve,

(D) solidifying the metal in the die, and

(E) ejecting the solidified casting and the residual metal from said shot sleeve.

2. The method in accordance with claim 1 in which the protective thermal barrier includes inserting a preformed refractory member into the shot sleeve.

3. The method in accordance with claim 1 in which the protective thermal barrier is applied by spraying a refractory slurry.

4. The method in accordance with claim 1 and the additional step of interposing a seal between the shot plunger and the metal receiving portion of the shot sleeve to protect the shot plunger from contact with the charged metal.

5. The method in accordance with claim 1 in which a shot sleeve having clearance with adjacent die parts is centered and restrained by closure of the dies.

References Cited UNITED STATES PATENTS 1,956,907 5/1934 Miller et a1. 164-120 2,582,260 1/1952 Kutik l64347 X 2,932,865 4/1960 Bauer 164--l13 3,447,593 6/ 1969 Nyselius 164-306 X 3,472,308 10/1969 Lauth 164123 X I. SPENCER OVERHOLSER, Primary Examiner J. S. BROWN, Assistant Examiner U.S. Cl. X.R. l64284 

